Seatback of a bench



FIG. 1 is a front-right-top perspective view of a seatback of a bench, showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left elevational view thereof;

FIG. 5 is a right elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof. 

The ornamental design for a seatback of a bench, as shown and described. 